1. Field of the Invention
The present invention relates to aqueous polyamide dispersions and methods for making and using such dispersions.
2. Background of the Related Art
Polyamide resins are well known as a class of resins, as are numerous methods for their preparation. Polyamide resins are typically manufactured by reacting a di- or poly- functional amine with a di- or poly- functional acid. Most of the commonly employed diacids and diamines yield polyamide resins which are essentially linear.
The properties of polyamide resins will vary considerably, depending upon the particular synthetic reactants employed. Polyamide resins which are prepared from relatively short chain diacids and diamines having, for example, 5-10 carbon atoms will tend to be relatively crystalline and have excellent fiber forming properties. These types of polyamide resins are typically referred to as nylons.
Polyamide resins are also prepared from relatively long chain polyfunctional acids and diamines. A particularly important class of polyamide resins of this type are referred to as polymerized fatty acid polyamide resins. The polymerized fatty acid polyamide resins are especially useful in products such as hot melt adhesives, water resistant coatings, and binders for printing inks because of their physical properties, including high strength, excellent flexibility, water and solvent resistance, and the ability to form smooth, non-tacky coatings and films.
The polyfunctional acids used in the preparation of polymerized fatty acid polyamide resins are derived from higher molecular weight unsaturated fatty acids by polymerization. In the polymerization process, the fatty acids having double bond functionalities combine to produce mixtures of higher molecular weight polymeric acids. Saturated fatty acids can also, under certain reaction conditions, polymerize to form polycerboxylic acids.
The polymerized fatty acid polyamide resins are, in turn, typically prepared by reacting one or more suitable diamines--most commonly relatively short chain diamines--with the polymerized fatty acid. Often, another diacid is also reacted to increase the softening point, tensile strength, or other properties. The polymerized fatty acid polyamide resins which are obtained tend to be more amorphous than the nylon types of polyamides resins and are generally more flexible. The differences in the physical properties of the polymerized fatty acid polyamide resins as compared to the nylon types of polyamide resins are related to the long chain length and structural variations of the polymerized fatty acid component.
With respect to aqueous ink compositions for use in the graphic arts the polyamide dispersions up to now have yielded ink films of insufficient quality. In particular, inks derived from aqueous polyamide dispersion suffer from blocking problems and poor adhesion to plastic substrates such as polyester, polyethylene and polypropylene, as well as poor mechanical stability.
Blocking can be defined as an undesirable adhesion between layers of printed material such as might occur under moderate pressure and/or temperature in storage or use, to the extent that damage to one surface is visible upon their separation. It is preferred that ink compositions, when printed onto a substrate, do not block below 100.degree. F.